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I don't know whether google apps scales to this size for enterprises but I would recommend google apps for domains as the spam, antivirus and the 20gb mailboxes are great value for $50 a year (I think), also supports IMAP so they can use pretty much any email client they like.
+1 for the poweredge, but do factor in that they are a little more power hungry than some other 1U servers. Some external hosts may charge you more if you are colocating it with them.
You may also want to take a look at the Microsoft Empower program, the entry requirements are a little more stringent but you get a lot more software (visual studio, windows server, msdn etc) for about £300.
If I were you I would look at the user guides/demos of the most popular web apps and take that as a guide. The ones that immediatly come to mind are 37signals (basecamp etc) and flickr, but there are loads of free/trial online apps that you could take a look at.
For some royalty free open source designs you can try www.oswd.org, you can do most of the changes you would need with notepad and then upload them using a free FTP program.
As said above you can go for visual studio with compact framework in VB.net or C# or you could use an off the shelf form builder like http://www.syware.com/
I have exited a contract which was had about 7 weeks left on it, giving the client the required 4 weeks worked notice. The client was very understanding and when out for drinks recently with people who still work there was told they would like me back. Obviously each situation is different but...